Filter

ABSTRACT

A filter consisting of two endless belts of filter material having runs pressed together, of means pressing a run of one belt against the adjacent run of the other, and of a feed device for introducing the slurry between the runs.

This is a continuation, of application Ser. No. 449,611 filed Mar. 11,1974, now abandoned, which is a division of application Ser. No. 253,178filed May 15, 1972, now U.S. Pat. No. 3,800,952.

BACKGROUND OF THE INVENTION

The filter of the invention is used for the removal of liquid fromsuspensions, sludges, and the like, particularly sludges derived fromthinner suspensions by thickening. Belt filters for this purpose havebeen described in German Patent Specification No. 1,277,203 which showsa feed device for the sludge and a power-driven, traveling,endless-screening belt guided over rollers and made of wire mesh orplastic mesh. The lower run of a pressure belt, also guided on rollersand situated above the screening belt, runs in the opposite direction tothe substantially horizontal upper run of the latter; the pressure beltextends over part of the length of the upper run of the screening belt,is pressed against the latter as it travels, the pressure forcesincreasing along the path of contact. Belt filters of this kind can beoperated continuously and, if necessary, without supervision; both thecapacity and the degree of dryness obtained in the press cake satisfymany requirements.

Another filtering system is described in Bulletin 700 of the firm ofSmith & Loveless, U.S.A., in which sludge taken from an inspissator ispassed on to a first traveling screening belt the end of which ejectspartly-drained product to a second screening belt following it. As thesecond screening belt effectively forms a prolongation of the first,this system neither saves over-all length nor gives any specialadvantage unless it is that of subdividing an already known belt intotwo separate belts. A leaflet entitled "GUVA" published by the GUVA AGVerfahrenstechnik und Apparatebau, of Rumlang, Switzerland, describes afilter tower in which use is made of two generally vertical movingfilter cloths which define between them a gap which narrows toward thebottom. Some materials to be filtered subject the filter cloths to thedanger of becoming seriously clogged; when sludge is used which in theinitial state is a thin liquid, this causes a comparatively highpressure in the vertical gap which prevents effective preliminarydrainage and results in a turbid filtrate. The moisture to be extractedcan emerge on both sides of the filter cloth, and the apparatus can beerected on a very small area. The height and space required, however,are considerable, and the constructional expense is on a level with thatinvolved in the previously-described type of belt filter.

The German Pat. No. 881,969 suggests, for the withdrawal of liquid suchas the juice from a pulp of fruit or the like, a number of systemsequipped with traveling sponge belts, which by their own suctionaleffect serve to extract the moisture or juice from the material fed tothe apparatus, the liquid being later pressed from the belt. In orderthat adequate lengths of these sponge belts can be accommodated, systemsare also described in which a number of these traveling absorbentendless belts are mounted one above the other. A knowledge of thesemultiple traveling absorbent layers (mainly used for the removal ofjuice or water from fruit pulp or foodstuffs) has nevertheless notproved an inducement to the technician concerned with the drainage ofclarification sludge, in the period of time that has passed since thepublication of the aforementioned patent, to apply the same technique tothis other sphere, which has different conditions.

It is, therefore, an outstanding object of the present invention toprovide a belt filter which will occupy little space and yet provideample active filter surface, so that high filter performance will beobtainable with only moderate space requirements and expenditure.

SUMMARY OF THE INVENTION

In general, the invention comprises a belt filter in which a pressurebelt and a screening belt are made from a petal or plastic mesh, and afeed device is mounted above the upper run of the pressure belt.

More specifically, according to the present invention a belt filter forremoving liquid from suspensions, sludges, and the like, comprises anendless, roller guided screening belt, having an upper run and a lowerreturn run, an endless roller guided pressure belt which is mountedabove the screening belt, and which has an upper run and a substantiallyhorizontal lower run. Means is provided for pressing at least a part ofthe length of the lower run of the pressure belt against at least a partof the length of the upper run of the screening belt, the pressureforces increasing along the contiguous parts of the belts, each of whichis formed at least in part of metal or plastic filter fabric. A feeddevice is situated above the upper run of the pressure belt and isarranged to transfer material to be deliquefied to the pressure belt,the material from the feed device being subsequently fed to the nipbetween the lower run of the pressure belt and the upper run of thescreening belt.

It has been found preferable for the feed device to be mounted above theupstream end of the upper run of the pressure belt, i.e. the end whichis the initial end as viewed in its direction of motion. The feed devicemay be formed as a feed tube whose lower end is situated above a feedapron located directly above the pressure belt. It is of advantage forthe pressure belt to be made long enough to ensure that the end of itslower run will project, in the direction of motion, beyond the end ofthe upper run of the screening belt.

BRIEF DESCRIPTION OF THE DRAWINGS

The character of the invention, however, may be best understood byreference to one of its structural forms, as illustrated by theaccompanying drawings, in which:

FIG. 1 is a vertical sectional view of a belt filter embodying theprinciples of the invention, and

FIG. 2 is a vertical sectional view of a second form of belt filter.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, power-driven reversing rollers 1 and 2 are mounted in a fixedframe (not shown). An endless first or screening belt 3 formed of wiremesh is stretched round the rollers 1, 2 in such a way that, when one orother of the reversing rollers 1 and 2 is driven, the belt travels in ataut condition and without sagging. A second frame, also omitted for thesake of simplicity, is mounted, preferably by a spring mounting, so thatit can be adjusted with respect to the first frame and carries reversingrollers 4 and 5 around which passes an endless second or pressure belt6, which is similarly made of wire mesh. The belts 3 and 6 have in thepreferred embodiment the same mesh number; however, they may differ inthis respect, the pressure belt 6 having, for instance, a higher meshnumber than the screening belt 3. The reversing rollers 1, 4 and 2, 5are not situated immediately one above the other but are staggered withrespect to one another in the longitudinal direction of the belts, sothat the pressure belt 6 extends to the right beyond the screening belt3, as seen in FIG. 1. The screening belt 3 projects beyond the pressurebelt 6 at the left hand end. Suspensions, sludge or the like, to bedrained, are conveyed to the upper strand of the pressure belt 6 via afeed pipe 7 and a feed apron 8 having side plates 9. The system is soconstructed that the sludge fed to the apparatus spreads out on the feedapron 8 and slowly moves over the edge of the latter onto the pressurebelt 6 at a speed equivalent to about half the linear speed of thatbelt. This ensures that the sludge is deposited on the pressure beltwithout causing damage and without the exertion of extraneous forces.Any flakes formed by the action of a flocculation agent are preservedand fine particles contained in the sludge or suspensions are depositedon the pressure belt without being washed through the holes by thoseextraneous forces. In a feed system of this kind the sludge is onlysubjected to the effect of gravity and of the water flowing out of thesludge, and all other forces (such as result from tangential movementsor the like) are avoided.

The sludge fed to the apparatus in this non-destructive manner moves tothe left on the upper run of the pressure belt when the latter is drivenand, in a pre-drainage zone following the feed apron 8 where the sludgeis again kept free of extraneous forces, a non-destructive drainageaction is obtained in which the sludge is further thickened and thusrendered mechanically more stable.

Towards the end of the upper run of the pressure belt 6, a number ofpressure rollers 10 in a preliminary pressure zone exert, perpendicularto the surfaces of the said pressure belt 6, forces increasing from onepressure roller to the next and intensifying the drainage effect. Duringthe passage through the pre-drainage zone, the sludge to be drainedacquires a consistency which enables the pressure rollers 10 to be usedand enables the perpendicular forces to be increased from one roller tothe next according to the increasing degree of drainage obtained,leading consequently to the consolidation of the sludge. An interceptiontrough 11, which catches and removes the water discharged through themesh of the belt 6, is provided below the pre-drainage zone and thepre-pressing zone.

At the end of the upper run the pressure belt 6, on rotation of thereversing roller 4, ejects the pre-drained sludge on to the projectingfree end of the screening belt 3, which introduces it into the gapformed between the upper run of the screening belt 3 and the lower runof the pressure belt 6. Between the pressure belt and the screening beltcomparatively intensive perpendicular forces are generated whichincrease along the path of contact in the direction of motion of thebelts. To enable these forces to be asborbed, the screening belt 3 andthe pressure belt 6 are supported by supporting rollers 12, 13respectively. The supporting rollers 12 and 13 are situated at equalpitch along the direction of motion of the belt but staggered in respectof one another by half the pitch. Within a first zone (the pressurezone), they are so situated with respect to one another that, althoughincreasing forces are exerted on the sludge present between the pressurebelt and the screening belt, both the pressure belt and the screeningbelt run in a generally straight stretched condition. An interceptiontrough 14 is mounted beneath the pressure zone to receive the dischargeof the clarified filtrate. The pressure zone above the interceptiontrough 14 is followed by a shearing zone which is situated above aseparate interception trough 16. Within this zone the supporting rollers12 and 13 come increasingly close together, so that the supportingplanes described by their supporting generatrics pass through oneanother and the runs of the screening belt and pressure belt passingbetween them travel in a zigzag course. The layer of sludge presentbetween the screening belt and the pressure belt is bent and pressedseveral times, so that, as a result of the resulting shearing forces,matrices of larger or smaller particles built up within the alreadylargely-drained sludge are caused to collapse, and the liquid in thecavities within is pressed out. Since in this process there is a strongtendency for smaller particules to be carried along with the filtrate,it is found advisable for the filtrate from the shearing zone to bepurified again instead of being discharged together with the clarifiedfiltrate from the troughs 11, 14.

At the end of the upper strand of the screening belt 3, i.e. in the areaof the roller 2, there is a chute 17, by which drained and pressed cake,produced at the end of the contact path between the screening belt 3 andthe pressure belt 6, is removed. At the end of the lower strand of thepressure belt 6 and at the beginning of the lower strand of thescreening belt 3, discharge devices 18 and 19 respectively directcleaning water onto the belts in order to clean them. The cleaning waterflowing off and through the belts is intercepted in troughs 20 and 21and conveyed together with the filtrate of the interception trough 16 toa pump 22 by which the water still containing solids is conveyed to afeed device 24 located above the pre-drainage zone. The water fed in atthis point is added to the sludge whose drainage has already commenced;the lower zones of sludge are already poorer in water and thusconsolidated. This layer constitutes an additional filter layer for thecontaminated water fed to it from the feed device 24. The pipe 23 fromthe pump 22 may also be directed to the feed pipe 7, so that the waterwhich contains suspended substances, i.e. the washing water and theshearing filtrate from trough 16, is delivered with the flocculatedsludge, the suspension, or the like.

An alternative arrangement is shown in FIG. 2. In order to simplify thediagram and avoid repetitions, parts similar to those already known fromFIG. 1 have been given similar reference numbers. The construction ofFIG. 2 is somewhat more compact than that of FIG. 1, since the screeningbelt 33 and the pressure belt 36 are more closely aligned lengthwisethan was the case in FIG. 1. Nevertheless, in order to ensure that thesludge pre-drained on the upper run of the pressure belt is conveyed tothe zones formed between the belts, i.e. the pressure zone and theshearing zone, a baffle 45 is provided behind the reversing roller 34and assists, by its shape, in the restratification of the pre-drainedcake. To obtain a further shortening of the construction, the dischargedevice 49 is provided above the upper run of the pressure belt. In FIG.2 the feed pipe 37 leads obliquely to the feed apron 38, in such a waythat the horizontal component in the feed of the sludge takes theopposite direction to the movement of the upper run. Particularly in thecase of intensively-flowing sludge with a high water content it has beenfound satisfactory to prolong the side plates 39 of the feed apron 36beyond the latter and to have them extend, for example, over practicallythe whole of the pre-drainage zone. In a similar fashion side plates maybe provided along the contact path of the belts.

The constructions described provide a number of far-reaching advantages.By the utilization of the pressure belt for drainage purposes the pathlength of the sludge in the apparatus is approximately doubled. Sinceparts of the path must be used for washing devices, feed aprons and thelike, and cannot be used for drainage, the relative increase in theactual length used for drainage is still greater. Moreover, since alongthe path of contact of the belts, liquid not only penetrates thescreening belt downwardly but can also be discharged upwardly throughthe pressure belt, the actual surface participating in the drainage ismore than trebled in comparison with known machines of similar type andof the same size.

According to the design and operation of the machine, therefore, eithermany times the previous capacity can be obtained, or the residualmoisture can be correspondingly reduced. In practice, the speed, contactpressures, and mesh widths are selected to ensure a considerableincrease in capacity combined with a more thorough removal of water,providing a drier cake than with machines of the usual type.

The improved effect, however, is not solely due to the increase in thesurface area utilized for drainage. Owing to the re-stratification ofthe pre-drained sludge in its transfer from the pressure belt to thescreening belt, the upper layer (where the drainage is less satisfactoryowing to it distance from the mesh of the pressure belt) is now locatedon the lower side and is thus in direct contact with the screening belt,so that its higher water content is rapidly reduced. At the same time,with the re-stratification brought about by the transfer from thepressure belt to the screening belt which is driven in the oppositedirection, the structure of the cake formed in the pre-draining of thesludge is altered in such a way that the discharge of water is furtherand considerably increased. These effects, which greatly assist theremoval of water, are obtained neither in the known method of transferto a second belt running in the same direction, nor in the method inwhich filtered product is ejected onto another belt inclined to theoriginal belt at an angle; it is only when it is ejected from the upperbelt to a strand of belting moving below it and in the oppositedirection that the effects are obtained in their entirety.

The possibility of discharging water from the cake through the pressurebelt, both during its passage through the pressure zone and during itspassage through the shearing zone, is of advantage not only because itincreases that area of the apparatus which permits passage of drainwater; it is now possible for the water within the lower vertical halfof the layer to be discharged downwards, while that within the upperhalf of the layer can be discharged upwards. Thus, the maximum length ofpath to be traversed by the water within the cake no longer correspondsto the full height of the latter, but only to half the height, and theresistance which resists the outflow of the water is considerablyreduced. Thus, the system avoids what occurred in earlier belt filterconstructions with a non-permeable pressure belt; the pressed cakedischarged at the end of the contact path was drained of water morethoroughly on the lower side than in its upper layers. The use of aperforated or meshed pressure belt with the possibility of dischargingwater both upwards and downwards results in a product in which theresidual moisture is not only less than in systems previously known butwhich is also far more evenly distributed. Even during the passage alongthe path of contact this system already provides considerableadvantages; hitherto, the pressure had to be increased comparativelyslowly, since the zones of the cake facing towards the pressure beltretained moisture longer and thus remained flowable for longer thanthose zones of the sludge adjacent the screening belt. In the presentbilateral drainage method this effect is absent, the removal of waterbeing far more even throughout the thickness of the layer, so that thepressure can also be increased to a greater extent along the pressurezone. This is because no account need be taken of layers still remainingflowable and liable to emerge from the belts at the side.

In practical operation it has been found that 95% of the solids in thewaste water emanate from the shearing zone or from the washing devices.In a drainage water guiding system such as shown in FIG. 1, therefore,the clarified filtrate occurring in the pipe 15 can be regarded asperfectly clean, and the solids appearing in the waste water are removedpractically in their entirety by the pipe 23 and recycled to thefiltering process.

Fundamentally a drainage operation of this kind, as far as the wastewater having the main residual solids is concerned, could be alsocarried out with belt filters of the customary type. It is found,however, that the pre-drainage zone available in such systems isgenerally far too short, so that the cake which has formed at the end ofthe pre-drainage zone can only be subjected, in the pressure zone andalso in the shearing zone, to limited and gradually increasing forcesalong the path of contact of the belts. The result is that the ejectedpressed cake is not sufficiently drained for most applications, or elseimpractically limited throughput speeds must be applied. A system of thekind described herein, utilizing the recycling of the shearing filtrateand possibly of the washing water, offers further advantages besidesthat of an almost completely clean clarified filtrate. Owing to the moreintensive wetting of the pressure belt by the liquid from the feeddevice 24, that belt undergoes far less contamination from solidparticles adhering thereto than the lower belt. It is, therefore,possible to dispense with rinsing devices altogether, or to operatethese periodically, or to subject them to less force, resulting both ina saving of water and in a smaller quantity of washing water having torecycled. In most operations, owing to the greater extent to which thecake is pressed, a further advantageous effect has become evident; thescreening belt 3 undergoes normal contamination, but the solid particlesadhering thereto are taken up by new press cakes, under the effect ofthe increased pressure, and are removed from the screening fabric, sothat in operation a stable state is obtained, with relatively littlecontamination, and a washing device can then be dispensed with. Theadoption of high pressures between the belts may prevent water drainingfreely through the screening fabric as a result of the contamination,but that is no disadvantage since particles of dirt on the belt aretaken up by the new cake, and the pre-drained cake fed on to thescreening belt no longer has any freely discharging water which wouldnecessitate, for more rapid discharge, a completely clean screening beltwith ample through-flow cross sections. Thus, for drainage and in viewof the effect of high pressure within the pressing zone and shearingzone, it is immaterial whether the screening belt, which is in any casecovered by the cake, was originally contaminated or not. The use of apressure belt constructed on the lines of a screening belt, i.e. the useof two screening belts, therefore, makes it possible, by contrast withthe use of a single screening belt and a water impermeable pressurebelt, to limit the consumption of rinsing water considerably, or todispense with it altogether, so that only the shearing filtrate has tobe recycled and the pre-drainage zone is subject to less recycle liquid,despite the repeated processing of solids-containing filtrate.

The residual water retained in the meshes of the pressure belt andparticipating on the self-cleaning effect of the latter also involvescertain disadvantages. After the removal of the pressure belt from thescreening cake at the end of the shearing zone, the water may bereleased from the belt and may wet the surface of the cake. While it istrue that this small amount of water in itself plays no appreciablepart, it is possible, as shown in FIG. 1, to have a roller 26, coveredwith a sponge-like absorbent material, run loosely over the pressedcake, so that the water present on the surface of the latter is removedby capillary action. A pressure roll 27, driven by or driving the roller26, and mounted above a discharge channel, presses water out theabsorbent material, so that it is continuously capable of taking upfurther water.

Further variants are possible. Particularly in the processing ofextremely sensitive sludge, for example, it may become necessary for thecontact pressures exerted by the pressure rollers 10 to be sensitivelyadjusted to low values. In this case they can be loaded by adjustablesprings or else mounted on rocker arms the torque of which can beadjusted by weights or possibly by counterweights. It is also possiblefor pressure rolls of the usual kind having a continuous surface, to bereplaced by perforating screening rolls which, in turn, can contributefurther to the removal of water. The arrangement of the supportingrollers 12 and 13 can also be modified; it is possible, for example, forthem to be adjusted independently. While high maximum pressures areobtained in the contact zone (when the rollers 12, 13 are directlyopposite each other), they are greatly reduced by the offset positionsshown in the example. Intermediate values for the distances, or anoffset amounting to half the pitch, also result in intermediate pressuremaximums. Furthermore, the apparatus is not confined to the use of asingle pressure belt and a single screening belt; the space availablecan be put to further use and the structure rendered less expensive, inaddition to increasing the filter performance, if one or more additionalbelts are provided above the pressure belt. In this case, two belts canbe located at a distance from one another, so that only their upper runacts as a filter; after sufficient consolidation of the filter cake,however, the sludge is subjected to additional pressure between twobelts.

In all cases, belt filters of this category will be considerablyimproved, both with regard to the capacity and with regard to the degreeof dryness obtained in the press cake delivered, by the use of pressurebelts with meshes made of metal wire or plastic filament, thesuspensions to be drained being fed on to the upper run of the pressurebelt, so that in practice the expense incurred can be reduced and theperformance increased at the same time.

It is obvious that minor changes may be made in the form andconstruction of the invention without departing from the material spiritthereof. It is not, however, desired to confine the invention to theexact form herein shown and described, but it is desired to include allsuch as properly come within the scope claimed.

The invention having been thus described, what is claimed as new anddesired to secure by Letters Patent is:
 1. A filter for removing liquidfrom a slurry, comprising:a. a first endless belt of filter fabricmounted on end rollers so as to form a horizontally-elongated loophaving an upper run and a lower run, b. a second endless belt of filterfabric mounted on end rollers above the first belt so as to form ahorizontally-elongated loop having an upper run and a lower run, c.means pressing the lower run of the second belt against the upper run ofthe first belt, d. a feed device situated at the upstream end of andabove the upper run of the second belt, the material from the feeddevice eventually entering the nip between the lower run of the secondbelt and the upper run of the first belt, which runs are driven in thesame direction, and e. means underlying the upper run of the secondendless belt to hold it substantially flat for conveying and preliminarydraining of the newly-introduced slurry.
 2. A filter as recited in claim1, wherein the feed device extends over the width of the second belt andtransfers material to the second belt at a rate dependent on the speedof the belt, and wherein a pressure roller is provided having aperforated screen cylindrical surface, the roller being positioned atthe downstream end of the upper run of the second belt, and pressingagainst that belt.
 3. A filter as recited in claim 1, wherein the feeddevice comprises a feed apron which is located above the upper run ofthe second belt and which is fed through a feed pipe.
 4. A filter asrecited in claim 1, wherein the downstream end of the lower run of thesecond belt extends beyond the end of the upper run of the first belt.5. A filter as recited in claim 1, wherein the second belt has a guideroller at the downstream end of its upper run and that guide roller issituated above the upper run of the first belt.
 6. A filter as recitedin claim 1, wherein the means for pressing together the first and secondbelts comprise sets of supporting rolls acting on the upper run of thefirst belt and the lower run of the second belt.
 7. A filter as recitedin claim 6, wherein in a first section of a path of contact of thebelts, the supporting rollers on opposite sides of the belt are situatedone above the other while in at least one second section they arestaggered with respect to one another in a horizontal direction.
 8. Afilter as recited in claim 6, wherein the supporting rollers which areassociated with the first belt are offset in relation to the supportingrollers associated with the second belt by an amount equal to one-halfthe distance between successive rollers, while in at least one firstsection of a path of contact between the belts the second belt and thefirst belt take a straight course, whereas in at least one furthersection, owing to the greater penetration of the supporting rollers,they take a zigzag course.
 9. A filter as recited in claim 1, wherein aseparate collecting trough is situated below the downstream section of apath of contact of the belts, and the discharge from that trough is ledvia a pump to the upper run of the second belt.
 10. A filter as recitedin claim 9, wherein the discharge from the pump is led to the feeddevice for the material to be deliquefied.
 11. A filter as recited inclaim 9, wherein the discharge from the pump is also led to a secondfeed device delivering on to the upper run of the second belt.
 12. Afilter as recited in claim 9, wherein a washing device is provided fordelivering washing water to at least one of the belts, and wherein meansis provided for collecting the washing water and feeding it to the pump.13. A filter as recited in claim 1, wherein an absorbent material ispressed against pressed effluent emerging from between the first beltand the second belt.
 14. A filter for removing liquid from a slurry,comprising:a. a first endless belt of filter fabric mounted on endrollers so as to form a horizontally-elongated loop having an upper runand a lower run, b. a second endless belt of filter fabric mounted onend rollers above the first belt so as to form a horizontally-elongatedloop having an upper run and a lower run, c. means pressing the lowerrun of the second belt against the upper run of the first belt, d. afeed device situated at the upstream end of and above the upper run ofthe second belt, the material from the feed device eventually enteringbetween the lower run of the second belt and the upper run of the firstbelt, which runs are driven in the same direction, and e. meansassociated with the upper run of the second endless belt to hold itsubstantially flat for conveying and preliminary draining of thenewly-introduced slurry, wherein a separate collecting trough issituated below the downstream suction of a path of contact of the belts,and the discharge from that trough is led via a pump to the upper run ofthe second belt.
 15. A filter for removing liquid from a slurry,comprising:a. a first endless belt of the filter fabric forming a loophaving a upper run and lower run, b. a second belt of filter fabriclocated above the first belt and forming a loop having an upper run anda lower run, c. means pressing the second belt against the first belt toform a pressure zone in which the belts are driven in the samedirection, d. a feed device situated at the upstream end of and abovethe upper run of the second belt, the material from the feed deviceeventually entering a nip between the first and second belts, andpassing through the pressure zone, and e. means underlying the upper runof the second endless belt to hold it substantially flat for conveyingand preliminary draining of the newly-introduced slurry, the materialbeing completely enclosed and carried by the belts all the way from thenip to the said pressure zone.
 16. A filter as recited in claim 15,wherein the feed device extends over the width of the second belt andtransfers material to the second belt at a rate dependent on the speedof the belt, and wherein a pressure roller is provided having aperforated screen cylindrical surface, the roller being positioned atthe downstream end of the upper run of the second belt, and pressingagainst that belt.
 17. A filter as recited in claim 15, wherein the feeddevice comprises a feed apron which is located above the upper run ofthe second belt and which is fed through a feed pipe.
 18. A filter asrecited in claim 15, wherein the downstream end of the lower run of thesecond belt extends beyond the end of the upper run of the first belt.19. A filter as recited in claim 15, wherein the second belt has a guideroller at the downstream end of its upper run and that guide roller issituated above the upper run of the first belt.
 20. A filter as recitedin claim 15, wherein the means for pressing together the first andsecond belts comprise sets of supporting rolls acting on the upper runof the first belt and the lower run of the second belt.
 21. A filter asrecited in claim 20, wherein in a first section of a path of contact ofthe belts, the supporting rollers on opposite sides of the belt aresituated one above the other while in at least one second section theyare staggered with respect to one another in a horizontal direction. 22.A filter as recited in claim 20, wherein the supporting rollers whichare associated with the first belt are offset in relation to thesupporting rollers associated with the second belt by an amount equal toone-half the distance between successive rollers, while in at least onefirst section of a path of contact between the belts the second belt andthe first belt take a straight course, whereas in at least one furthersection, owing to the greater penetration of the supporting roller, theytake a zigzag course.
 23. A filter as recited in claim 15, wherein aseparate collecting trough is situated below the downstream section of apath of contact of the belts, and the discharge from that trough is ledvia a pump to the upper run of the second belt.
 24. A filter as recitedin claim 23, wherein the discharge from the pump is led to the feeddevice for the material to be deliquefied.
 25. A filter as recited inclaim 23, wherein the discharge from the pump is also led to a secondfeed device delivering onto the upper run of the second belt.
 26. Afilter as recited in claim 23, wherein a washing device is provided fordelivering washing water to at least one of the belts, and wherein meansis provided for collecting the washing water and feeding it to the pump.27. A filter as recited in claim 15, wherein an absorbent material ispressed against pressed effluent emerging from between the first beltand the second belt.